Java Scanner nextDouble() Method

The nextDouble() method in Java, part of the java.util.Scanner class, is used to retrieve the next token from the input as a double value. This method is useful for reading and processing floating-point values from the input.

Table of Contents

1. Introduction
2. nextDouble() Method Syntax
3. Understanding nextDouble()
4. Examples
   • Basic Usage
   • Handling Input Errors
5. Real-World Use Case
6. Conclusion

Introduction

The nextDouble() method returns the next token from the scanner's input as a double. This method is useful when you need to read and process floating-point values, which represent decimal numbers.

nextDouble() Method Syntax

The syntax for the nextDouble() method is as follows:

public double nextDouble()

Parameters:

• This method does not take any parameters.

Returns:

• The next token as a double value.

Throws:

• InputMismatchException: If the next token does not match the double regular expression, or is out of range.
• NoSuchElementException: If no more tokens are available.
• IllegalStateException: If the scanner is closed.

Understanding nextDouble()

The nextDouble() method retrieves the next token and converts it to a double. If the token cannot be interpreted as a double, an InputMismatchException is thrown.

Examples

Basic Usage

To demonstrate the basic usage of nextDouble(), we will create a Scanner object and use it to read double values from a string.

Example

import java.util.Scanner;

public class NextDoubleExample {
    public static void main(String[] args) {
        String input = "10.5 20.5 30.5";

        // Create Scanner object in try-with-resources to ensure it closes automatically
        try (Scanner scanner = new Scanner(input)) {

            while (scanner.hasNextDouble()) {
                double value = scanner.nextDouble();
                System.out.println("Double value: " + value);
            }
        } // Scanner is automatically closed here
    }
}

Output:

Double value: 10.5
Double value: 20.5
Double value: 30.5

Handling Input Errors

This example shows how to handle errors when the input token cannot be interpreted as a double.

Example

import java.util.InputMismatchException;
import java.util.Scanner;

public class HandleInputErrorsExample {
    public static void main(String[] args) {
        String input = "10.5 abc 20.5";

        // Create Scanner object in try-with-resources to ensure it closes automatically
        try (Scanner scanner = new Scanner(input)) {

            while (scanner.hasNext()) {
                try {
                    double value = scanner.nextDouble();
                    System.out.println("Double value: " + value);
                } catch (InputMismatchException e) {
                    System.out.println("Invalid input: " + scanner.next());
                }
            }
        } // Scanner is automatically closed here
    }
}

Output:

Double value: 10.5
Invalid input: abc
Double value: 20.5

Real-World Use Case

Reading Configuration Data

In real-world applications, the nextDouble() method can be used to read and process floating-point configuration data from a file.

Example

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ConfigParser {
    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(new File("config.txt"))) {
            while (scanner.hasNext()) {
                try {
                    double value = scanner.nextDouble();
                    System.out.println("Configuration value: " + value);
                } catch (InputMismatchException e) {
                    System.out.println("Invalid input: " + scanner.next());
                }
            }
        } catch (FileNotFoundException e) {
            System.out.println("File not found: " + e.getMessage());
        } // Scanner is automatically closed here
    }
}

Output (Assuming config.txt contains valid and invalid double values):

Configuration value: 10.5
Invalid input: abc
Configuration value: 20.5
...

Conclusion

The Scanner.nextDouble() method is used to retrieve the next token from the input as a double value. This method is particularly useful for applications requiring floating-point input values. By understanding and using this method, you can efficiently parse and handle double input data. Always close the Scanner using try-with-resources to ensure proper resource management.